you could go to a home improvement store and get a good quality paint & varnish remover(rinse able Type),Follow directions then sand prime and then paint with a good quality oil base enamel
Another option is find a auto body shop or a sand blasting shop and have them remove the old paint sand blasting is better as it will remove all the old paint and scale, It smooths small imperfections also
A sand blasted metal needs to be primed at once